The invention relates generally to the recovery of individual components from mixtures of different components and, more particularly, to the recovery of high-purity aromatic substances from hydrocarbon mixtures containing both aromatic and non-aromatic components.
A process for the recovery of high-purity aromatic substances from hydrocarbon mixtures containing aromatic and non-aromatic components in any relative proportions is already known. The hydrocarbon mixture is subjected to a liquid-liquid extraction and/or an extractive distillation using an aqueous solvent which is selective towards one of these components. The bottom product or extract phase formed in the extraction apparatus, which consists mainly of the aromatic component and the solvent, is withdrawn and then introduced into a separating column or distillation column wherein the extract phase is separated into an aromatic fraction and a solvent fraction. The top product or raffinate phase formed in the extraction apparatus, which consists mainly of the nonaromatic component, is also withdrawn from the extraction apparatus.
The known process outlined above is, in practice, used with many variations and with a large number of different types of aqueous, selective solvents. The addition of water to the solvent serves, among other things, to improve the selectivity of the solvent. As an example, a combined process is known from the German published application No. 2,040,025 wherein a liquid-liquid extraction is followed by an extractive distillation. The selective solvent used here is a mixture of morpholine and/or N-substituted morpholines with water. The water content of the solvent for the liquid-liquid extraction lies between 2 and 15% by weight whereas the water content of the solvent for the extractive distillation is up to 8% by weight.
When using aqueous solvents, however, particular problems arise in the operation of the separating column wherein the solvent and the aromatic component are separated which do not arise when non-aqueous solvents are used and where, consequently, it is only necessary to drive off the solvent, i.e. where no water must be driven off. In this connection, it must be taken into account that the aromatic component of the extract phase has to be separated as completely as possible, for example, so that at most about 0.1% by weight is left behind, in order to impart good efficiency to the process. Although it is true that the water content of the aqueous solvents provides for a lower bottom temperature in the separating column as compared to the case where non-aqueous solvents are used, it has, on the other hand, been frequently observed that the operation of the separating column becomes irregular or fluctuates when using aqueous solvents. The trays of the column become partially flooded or inundated and it is not possible to maintain either the bottom temperature or the column temperature constant.
A further problem which arises in the separating column when aqueous solvents are used is due to the fact that the possibility of hydrolysis of the solvent exists. Of course, very significant differences exist between the various solvents in this regard. However, because of the severe temperature requirements imposed upon the solvents during the operation of the separating column, even very stable solvents will undergo a certain amount of hydrolysis, albeit small. Thus, during the operation of the separating column, it is necessary to insure to as great an extent as possible that the degree of hydrolysis of the solvent is held to a minimum and that the products of the hydrolysis do not contaminate the aromatic fraction withdrawn at the top of the separating column.
It will, therefore, be seen that improvements in the recovery of high-purity aromatic substances from hydrocarbon mixtures are desirable.